The present invention relates to wireless communications, more specifically, to a method and apparatus for detecting users at a time division multiple access (xe2x80x9cTDMAxe2x80x9d) receiver.
Prior art TDMA cellular/personal communications system (xe2x80x9cPCSxe2x80x9d) receivers generally include a series of processing stages for exploiting the features of a one dimensional signal stream such as the output signal of a single analog to digital converter.
With reference to FIG. 1, a receiver may include in its processing stages a spatial filter 11, a polarization filter 13, a channelizer filter 15, and a multi-user detection filter 17. Each stage acts as a user-population filter to separate the users in the receive environment based on polarization, spatial, and frequency characteristics as illustrated by the concentric subsets shown in FIG. 2.
The performance of each stage is a factor in designing a workable receiver architecture. Cost and complexity are dominant features. Frequently, receiver design is also constrained by the number of available observation array elements due to limited access to the geographic areas selected for deploying antenna assets.
Multi-user detection is the last stage of the receiver. A one-dimensional channelized signal stream which contains a cluster of cochannel signals is fed to the last stage. The signal stream may be TDMA-staggered (e.g., GSM, IS-54, IS-136) but contains multiple signals within a common time-slot.
In dense wireless environments, cochannel interference and intersymbol interference are significant problems for the detection of multiple users. Improving the performance of the last stage in detecting users may simplify the processing burden on preceding stages.
Generally, cochannel interference is caused by a multitude of other signals impinging upon the communication receiver. For TDMA systems, numerous techniques exist for cancelling cochannel interference. One known cancellation technique attempts to eliminate interference through a filtering process using the demodulation-remodulation technique and another method uses the cyclostationary signal processing technique.
The demod-remod technique uses successive stages of cancellation to eliminate one signal at a time. One problem with this technique is that signals must be widely spaced in power from each other or the error-rate is too high to perform effective remodulation/cancellation. Acquisition time may also be an issue because of the time needed to allow each stage to pull in.
Under certain conditions cyclostationary signal processing known as frequency-shift filtering (xe2x80x9cFRESHxe2x80x9d) can be used to separate interfering signals. The disadvantages of FRESH are that the symbol rates and carrier-frequencies must be distinct, i.e., the frequencies must be known with high accuracy and the channel transfer function must be known for each signal. Finally, the pulsed nature of TDMA is not very amenable to the FRESH architecture.
Another known cancellation technique does not attempt to eliminate the interference, but jointly detects all signals simultaneously. Joint maximum likelihood sequence estimation (xe2x80x9cJMLSExe2x80x9d) is an example of a joint detection technique.
JMLSE is known in the art to be the highest performing technique for removing co-channel interference. JMLSE directly embraces the signaling probability functions and is capable of exploiting carrier-frequency and symbol-rate differences. However, the main drawback of JMLSE is its complexity.
Intersymbol interference (xe2x80x9cISIxe2x80x9d), also known as multipath interference, as opposed to cochannel interference is generally caused by multiple signal propagation paths. These multiple paths result from the signal being scattered by a myriad of objects prior to reaching the receiver. The resulting interference is a form of self-interference which lowers the performance of the system.
Modern communication theory has devised multiple techniques for combating ISI-induced performance loss. In sequential order of best-to-worst performance, they are: the matched-filter bound technique, the maximum likelihood sequence estimator (MLSE), the decision feedback equalizer (xe2x80x9cDFExe2x80x9d) and the linear equalizer.
DFE""s may be either fractionally-spaced or symbol-spaced, where the fractionally-spaced equalizer is higher performing. DFE operation is often several dB better than a linear equalizer which may amplify noise when channel nulls are present.
The matched-filter bound technique is not practical in application, because isolated pulses are sent at a relatively-slow symbol rate.
The MLSE receiver is theoretically the top-performing technique. However, MLSE is usually too complex to be used in many applications. Nevertheless, most TDMA cellular receivers use MLSE because the multipath spread usually only extends across a few symbols.
In systems employing an MLSE receiver, a transmitter 10 such as shown in FIG. 3 may generate a 3-tap channel 4-ary pulse-amplitude-modulation (xe2x80x9cPAMxe2x80x9d) signal with a transmit symbol Ak at time k taken from the signal set {xe2x88x923, xe2x88x921, 1, 3}. The noise-free signal arriving at the receiver can be represented by a finite state machine (xe2x80x9cFSMxe2x80x9d) as shown in FIG. 4. Since there are only two delay registers 11 and 12, the FSM only remembers the last two inputs, Ak and Akxe2x88x921 (Ak, Akxe2x88x921) which define the state. The channel transfer function for this example is:
H(z)=xe2x88x920.5+zxe2x88x921xe2x88x920.25zxe2x88x922xe2x80x83xe2x80x83(1)
Given a new input Ak+1, a new output voltage is generated and a new state results as shown in FIG. 5. The output voltage and new state are a function of the current state and the new input. Since the new input Ak+1 can be one of four values, each state has four exiting branches.
Generally, given an M-ary constellation with an (L+1)-tap channel, the number of states is equal to ML, and the number of branches entering and exiting each state is M.
For each noisy received sample, M*(ML) many branch metrics are computed and added to the preceding state metric. All branches entering the output state are eliminated except for the one corresponding to the minimum cumulative distance metric. The output state with the best (i.e., smallest) metric is used to make a symbol decision by tracing back into the trellis a certain depth (the xe2x80x9ctraceback depthxe2x80x9d). The traceback depth is usually about five times the channel length.
Therefore, the complexity is roughly proportional to ML. The complexity can become prohibitively large for implementation with high-dimensional modulations where M is large (e.g.,M=16) and channel impulse responses which are long (e.g., L=8). Thus, both the storage demands and the operation complexity may become unmanageable.
It is object of the present invention to provide a novel joint MLSE/DFE technique for user detection in a TDMA system.
It is yet another object of the present invention to provide a novel method and-apparatus for detecting multiple users in a TDMA system with an MLSE operating on a signal stream having a cancelled postcursor portion.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims when read in connection with the appended drawings, and the following detailed description of the preferred embodiments.